The reports of Kiely and Chen 1,2 describe a process for the preparation of stereoregular head, tail-poly(alkylene D-glucaramides) (a class of polyhydroxylpolyamides, PHPAs). The alkylenediamine monomer units reported contained C2, C4, C6, C8, C10, & C12 carbons, respectively. The reported process generates polyamides with number average molecular weights (Mn) of 1,200 (C2), 2,100 (C4), 2,440 (C6), 1,600 (C8), 4,400 (C10), 3,800 (C12), respectively. These Mn values correspond to weight average molecular weights (Mw) of ca. 2,400-9,000, respectively. In general, the reported process does not produce polymers whose molecular weight values are high enough to classify the polymers as high polymers. As indicated by Billmeyer (3, p. 17), “Number-average molecular weights of commercial polymers usually lie in the range of 10,000-100,000, although some materials have values of MN 10-fold higher, and others 10-fold lower. In most cases, however, the physical properties associated with typical high polymers are not well developed if MN is below about 10,000”. Likewise, as stated by Billmeyer (3, p. 4), “Most high polymers useful for plastics, rubber, or fibers have molecular weights between 10,000 and 100,000”. It is clear that the molecular weight distributions of the above reported polymers are low by conventional “polymer science” wisdom. Thus, it is important that significant increases in the molecular weights of PHPAs as reported above would likely be necessary to make them useful as polymeric materials.
In order to achieve sufficiently high molecular weight condensation polymers of the type described, an essential requirement is that the (molar) stoichiometry of the appropriate reacting diacid and diamine components be 1:1. According to Allcock and Lampe, any deviation from this stoichiometry will severely limit the molecular weights of the resultant polyamides.(4, pp 274-275) The stoichiometry issue has been adequately addressed by forming an “amino acid-monoamide monomer”,1,2,5 a stereospecific monomer with the C-6 of the glucaryl unit in an amide bond with one of the amine functions from a primary diamine. The termini of this monomer were the C-1 carboxyl group (esterified) of the glucaryl unit and the remaining terminal primary amine unit of the starting diamine, respectively. A second factor that is essential in creating high polymers in a solution condensation polymerization in which the polymer precipitates from solution, is that the polymer formed must first reach a sufficiently high molecular weight before it precipitates from solution. In the above, the polymerizations were carried out in methanol solution, and the product polymers isolated by filtration.1,2 Consequently, the size of the polymers that were produced were limited by their solubility in methanol solution and have low reported molecular weights as indicated above.
Using solvents that keep growing polymers in solution longer than is the case with methanol, or other simple protic solvents, should produce higher molecular weight polyamides directly. Polymerization is dimethylsulfoxide (DMSO) is a much better solvent for dissolving polyamides than is methanol. Furthermore, DMSO has been previously reported as a solvent to make polymers derived from glucaric acid.6,7 However, while it is clear that DMSO is a solvent for preparing PHPAs by virtue of the solubility of the starting monomers and resulting polyamides, it is not clear how well the polymerizations proceed in DMSO as compared to protic solvents such as methanol. In our hands, use of DMSO alone as a polymerization solvent is not satisfactory, i.e., most of the polyamides formed are very soluble in DMSO and product isolation is much more difficult than when methanol or another protic solvent is the reaction medium. Furthermore, in spite of the high solubility of the polyamide products in DMSO, we discovered that polymerizations are much slower in DMSO alone as solvent compared to methanol and other protic solvents. In addition higher molecular weight polymers are not produced as required.
A reasonable compromise considered was to carry out the polymerizations in a mixture of an aprotic solvent with a high dielectric constant (e.g. dimethylsulfoxide) and an alcohol (e.g., methanol). However, the predicted results were not realized and the resulting polyamide produced had only a marginally higher molecular weight and corresponding lower isolated yield than was achieved when methanol alone was used as the solvent.
Another consideration that had to be taken into account for making high molecular weight stereoregular polyamides of the type described, was avoiding reaction conditions that would break the original “amino acid-monoamide monomer” monoamide bond to give an amine and esterified carbohydrate diacid units, a condition that would lead to stereochemically random polyamides. It was discovered in our hands that following the procedure described,2 significant alcoholysis of the monoamide bond of the starting sodium salt of the amino acid occurred, leading to esterified diacid plus diamine in mixture with the desired esterified monoamidoamino acid. Polymerization of this monomer mixture leads to significantly decreased stereoregular polymer formation. To achieve the desired higher molecular weight materials, and to reduce the possibility that the stereoregularity of the polymers was not seriously compromised, several unique adaptations to the published process had to be undertaken.
All patents, patent applications, provisional patent applications and publications referred to or cited herein, or from which a claim to benefit of priority has been made, are incorporated by reference in their entirety to the extent they are not inconsistent with the explicit teachings of the specification.